1. Field of the Invention
This invention is concerned with improvements in or relating to metal spraying devices.
2. Description of the Prior Art
Metal spraying devices are often of the kind comprising feed means for simultaneously feeding two metallic wires or rods (hereinafter for convenience merely referred to as "wires") through a pair of guide tubes which converge forwardly of the direction of wire feed, means for connecting an electrical potential across said wires to cause an arc to be established between the wires where they emerge from said guide tubes so as to melt the wires, and means for feeding a stream of compressed air or other non-combustible gas (hereinafter referred to for convenience as an "air jet") relative to the wires and the arc therebetween so as to cause molten metal droplets to be carried from the arcing zone on the air jet from the device. Such a metal spraying device is hereinafter referred to as being of the kind specified, and an example of such a device is described in the Specification of our U.K. Pat. No. 1,346,054.
One of the difficulties associated with metal spraying devices of the kind specified is that of guiding the two wires forwardly and in accurate relationship one to the other, so that the position and length of the arc between the wires where they emerge from the guide tubes remains essentially constant. It will be understood that the length of the arc is normally quite small, and that a very small discrepancy between the relative positions of the leading ends of the two wires could quite rapidly cause the arc to lengthen to an extent such that it would be extinguished by the air jet, or alternatively to cause the two wires to come into contact and thus extinguish the arc.
The direction of the air jet must also be precisely related to the arcing zone in order to ensure that the molten metal will be propelled forwardly from the device in a stream of suitably divided droplets. It will be appreciated that displacement of the arcing zone will result in incorrect positioning of the air jet, relative to the arcing zone, leading to unsatisfactory formation of the spray stream.
In known metal spraying devices wires are fed through two tubular electrically conductive guide tubes which extend to positions on either side of the air jet nozzle, which is secured to the device separately from the guide tubes. With this arrangement the capability of the guide tubes to provide support for the wires close to the arcing zone is limited, due to the physical size necessary for the guide tubes and nozzle, together with the difficulties associated with the electrical insulation of the components one from another. Thus, any kinks or similar irregularities in the wires as they emerge from the guide tubes can cause a significant displacement of the arcing zone.
Furthermore, under repeated operation, considerable wear occurs in the guide tubes due to the rubbing action of the wires against the tubes, and due to the eroding effect of minute electrical arcs which occur from time to time between the wire and the walls of the guide tubes. Such minute electrical arcs occur due to failure of the wires to be maintained in good electrical contact with their associated guide tubes, and may possibly be caused by oxide films on the wires, and/or the presence of dust or other foreign matter carried into the guide tubes by the wires. The excessive clearance between the wires and the guide tubes resulting from wear further reduces the support and guiding capability, which rapidly promotes intermittent arcing and the inclusion of unmelted wire ends in the coating being sprayed.
Furthermore, the need to maintain the air jet nozzle electrically insulated from the guide tubes often necessitates the distance between the nozzle and the arcing zone being substantial: thus, either the adverse effects of turbulence developed in the air jet in its flow from the nozzle to the arcing zone must be tolerated, or special provision to minimise such effect must be made.
It is one of the various objects of this invention to provide a metal spraying device in which high quality of metal spray is achieved consistently, and/or relatively inexpensively.